High temperature resistant coating compositions

ABSTRACT

An aqueous phosphoric bonding solution consists essentially of phosphoric acid, a source of magnesium ions, and a leachable corrosion inhibitor. The bonding solution is stable with respect to inorganic metal particles, such as aluminum, which are admixed to the bonding solution for the preparation of a coating slurry. Metal parts coated with the coating compositions have very satisfactory properties such as heat and corrosion resistance.

BACKGROUND

Compositions comprising phosphoric acid and aluminum metal are wellknown for use in protecting metallic surfaces such as ferrous surfacesfrom corrosion. In such coating compositions, particulate metallicmaterial, such as aluminum flake and/or powder, is combined with aphosphoric acid bonding solution to form a coating composition which isthen applied to the metallic surface being treated. After application ofthe coating to the substrate, it may be heated to a first temperature,generally upwards of 500° F. (260° C.), until the coating is renderedessentially water insoluble. Then the coated surface may be cured at asecond temperature, generally above 1000° F. (538° C.) to form the finalprotective coating.

A problem which arises in this coating process is that when theparticulate metallic material is combined with the phosphoric acidbonding solution, the acidic bonding solution can react with themetallic material. Such reactions can be very violent, causing the flakeand/or powder to burn or even explode, or less violent, simply resultingin the conversion of the metallic material into various salts. In eithercase, such reactions interfere with the formation of suitable protectivecoatings.

Allen U.S. Pat. No. 3,248,251 describes coating compositions consistingessentially of a slurry of solid inorganic particulate material (such asaluminum) in an aqueous acidic bonding solution containing dissolvedmetal chromate, dichromate or molybdate, and phosphate. It was foundthat the addition of chromates or molybdates to the acidic bondingsolution effectively passivated the solution toward aluminum andinhibited the oxidation of metallic aluminum, allowing particulatealuminum to be combined with the bonding solution without theundesirable chemical reaction between the acidic solution and thealuminum. These so-called Allen coatings have been successfully used toprovide high quality coatings which protect ferrous metal alloy surfacesfrom oxidation and corrosion, particularly at high temperatures.

While chromates and molybdates have been used successfully to reduce thereactivity of the aluminum in such coating compositions, the use ofchromates and molybdates has become a problem because of environmentalconsiderations. Chromates and molybdates are generally considered to betoxic substances. Hexavalent chromium is rated as a carcinogen. It istherefore desirable to avoid the use of solutions of such salts, or atleast to reduce their use. For this reason, it has become desirable todevelop a phosphate/aluminum coating composition which requires littleor no chromates or molybdates to control the reactivity between theacidic phosphate bonding solution and the particulate aluminum addedthereto. The coating compositions should protect ferrous metal alloysurfaces from the oxidation and corrosive environmental conditions,especially at high temperatures, at least as well as the Allen-typecoatings.

Some efforts have been made to overcome the environmental problemassociated with chromates and molybdates. Stetson et al. U.S. Pat. No.5,242,488 describes a coating composition for ferrous alloys which doesnot require either chromates or molybdates to control the reactionbetween the bonding solution and the powdered aluminum. The compositionconsists essentially of a slurry mixture of a bonding solution andaluminum powder. The bonding solution consists essentially of water,phosphoric acid (H₃PO₄), and aluminum ions in solution in an amountsufficient to substantially equilibrate the bonding solution withrespect to aluminum metal pigment. The bonding solution component of thecoating composition requires sufficient aluminum ions in solution sothat it is substantially equilibrated with respect to aluminum metalpigment, i.e., that the amount of aluminum ions in solution besubstantially at the saturation point and therefore, essentially inertwith respect to any subsequent addition of aluminum metal pigment.

Stetson discloses using magnesium (either MgO or MgCO₃) to at leastpartially neutralize the aqueous phosphoric acid mixture, either beforeor after equilibration of the mixture with aluminum. A more recentpatent to Stetson et al., U.S. Pat. No. 5,279,649, describessubstantially the same compositions to which V₂O₅ has been added toproduce vanadate ion, adding another inhibitor to the aluminumequilibrated mixture. Further, Stetson et al. U.S. Pat. No. 5,279,650discloses a seal coating composition of the coating disclosed in the'649 patent which also contains iron oxide (Fe₂O₃) powder. All threecoating compositions are designed to avoid the use of the chromium andmolybdenum ions and require the bonding solution to be equilibrated withrespect to further additions of aluminum. The addition of V₂O₅demonstrates the addition of a toxic substance, listed on the OSHAextremely hazardous substance list.

Environmentally friendly coating compositions are described in commonlyassigned Mosser et al. U.S. Pat. No. 5,478,413, which discloses anaqueous solution of phosphoric acid, one or more sources of magnesiumions, and a source of aluminum ions and/or sources of zinc and borateions. The amount of aluminum in solution is in an amount less than thatnecessary to reach the saturation point, thus, less than the amountnecessary to equilibrate the bonding solution with respect to additionalaluminum. The bonding solutions are non-reactive with respect toparticulate aluminum for at least one hour.

SUMMARY

In one aspect, a bonding solution for use in forming a ferrous alloycoating composition consists essentially of phosphoric acid, a source ofmagnesium ions, and a leachable corrosion inhibitor. The bondingsolution has a pH in the range of about 2 to about 4.5, usually fromabout 2.5 to about 3.5, and often from about 2.7 to about 3.3. To obtainthe desired pH, additional amounts of acid or base may be added to thecomposition as needed. To lower pH, for example, a water-solublephosphoric acid or acid salt such as magnesium dihydrogen phosphate maybe added.

In another aspect, a coating composition comprises the bonding solutionand particulate metallic material such as aluminum. The particulatemetallic material may be of any suitable form, such as powder, flake, ora combination of powder and flake.

In another aspect, a method of protecting a substrate comprises applyingthe coating composition to a metal surface of the substrate, and heatingthe coated substrate to cure the coating composition.

The bonding and coating compositions usually are free or substantiallyfree of chromium, especially hexavalent chromium and molybdate and othertoxic metals, like nickel and vanadium. Though free or substantiallyfree of such objectionable metals, the compositions are stable forperiods of time adequate to apply the coating, especially for periodsexceeding 1 hour, usually more than 4 and often more than 8 hours. Somecompositions are stable for several days and remain liquid for manyweeks.

The coatings are very satisfactory, in general meeting or exceeding thestandards of the Allen coatings in terms of resistance to oxidation andcorrosion, especially at high temperature. The bonding and coatingcompositions exhibit such stability and performance characteristicswithout the need for adding sources of zinc ions and/or aluminum ions asdescribed in U.S. Pat. No. 5,478,413, thus simplifying the compositionsand their manufacture. The coatings are especially well suited forturbine compressor airfoils, like blades, vanes, stators, and the like.

The bonding solution may be prepared by first preparing a bindersolution by combining water, phosphoric acid, a source of magnesiumions, and optionally a source of borate ions. The leachable corrosioninhibitor may then be added to the buffered binder solution to preparethe bonding solution. The leachable corrosion inhibitor and/or othercomponents of the bonding solution may have low or reduced solubility ormiscibility in water or in the aqueous phosphoric acid. Such lesssoluble or miscible components may be present in emulsion or othernon-solution form. Such terms used herein as “aqueous bonding solution”and “bonding solution” are thus intended to include compositions inwhich one or more components may not be fully dissolved, but may beemulsified or dispersed or in another form. This statement applies tocomponents described herein as well as others not described.

A coating composition may be prepared by combining the bonding solutionand a solid particulate metallic material, such as aluminum powder.Instead of or in addition to aluminum, other metal particles may beused, such as those disclosed in the above-referenced Allen patent. Thecoating composition may contain other components conventionally used inthe coating industry, such as non-metallic pigments like alumina,zirconia, ceria, and/or other mixed metal oxides. The coated ferrousparts exhibit very satisfactory properties, generally equivalent orbetter than those achieved by the coatings described in the Allen andStetson patents.

DETAILED DESCRIPTION

The aqueous bonding solution contains phosphoric acid, a source ofmagnesium ions, and a leachable corrosion inhibitor. The pH of bondingsolution may be adjusted to the range of about 2 to about 4.5, usuallyfrom about 2.5 to about 3.5, and often from about 2.7 to about 3.3.

The bonding solution is stable, that is unreactive or substantiallyunreactive (or inert) to metallic (e.g., aluminum) particles addedsubsequently. In a reactivity test as described below, the coatingcomposition exhibits no or essentially no visible reaction when aluminumparticles are admixed to the bonding solution for at least up to onehour, often up to 4 hours, and in some instances up to 8 hours or more.

The magnesium ions in the bonding solution may be supplied by way of anyconvenient source such as in the form of magnesium carbonate, magnesiumoxide or hydroxide, magnesium metal, or combinations thereof. Themagnesium dissolves in the phosphoric acid forming the metal ions andwater and/or gas. The amount added alone or in combination with theother compounds should be sufficient to bring the pH within the desiredrange or somewhat below or above the range so that upon addition of theother compound(s) the pH will be within the desired range.

Other suitable magnesium compounds which can serve as a suitable sourceof magnesium ions are listed in the Handbook of Chemistry and Physics,87th Ed. CRC Press, Inc. Boca Raton, Fla., Editor David R. Lide, in theChapter on Physical Constants of Inorganic Compounds (“Handbook”), whichis incorporated herein by reference.

The bonding solution also includes one or more leachable corrosioninhibitors (sometimes also referred to as leachable pigments). Theleachable corrosion inhibitor is capable of inhibiting or passivatingthe corrosion of the metal substrate. The leachable cation(s) may insome instances serve to passivate or stabilize the aluminum metalpowder.

Pigments are often used in the paint industry to provide color oropacity, or to modify surface hardness, wetting properties, or othercharacteristics of polymer films. Pigments also may be used to providecorrosion resistance as barriers, e.g., flake-like pigments. As anotherexample, anticorrosive pigments are selectively leached of specific ionsor neutral compounds that alter the rate of corrosion of metallicsubstrates. For example, strontium chromate and zinc chromate leachsmall amounts of chromate ion, which effectively passivates most metalsin neutral aqueous solutions. In general, such pigments are used only inporous primers which are topcoated with an effective barrier polymerfilm that seals and protects the primer from leaching except in thosesituations when the film is damaged or permeated.

Unlike paints, inorganic aluminum pigmented slurry coatings based onacidic phosphate binders have no organic carbon based polymer as thefilm former in the coating. The slurry coatings instead are water-basedand are very porous yet they are often used without a topcoat or sealer.The addition of neutral modified phosphate pigments as corrosioninhibitors creates several unexpected improvements, such as (1) the potlife (mixed usable coating life) of the coating is extendedsignificantly; (2) the chemical attack of the acid phosphate bindersolution on steel substrates is reduced or eliminated; and (3) theperformance of the applied aluminum coating under extreme conditions(salt spray, high temperature cycling, etc.) is enhanced. While it isnot known specifically how these pigments are improving performance itis believed that small amounts of zinc, aluminum, and/or other ions areleached out of the coating after mixing and during exposure. Thefollowing table provides non-limiting examples of leachable corrosioninhibitors that may be used. These fall into the classification of metalorthophosphate or metal polyphosphate.

Designation Manufacturer Chemical Name ZPA Heubach Zinc aluminumorthophosphate ZAP Wetech Zinc aluminum tripolyphosphate KWhite 105Tayca Aluminum tripolyphosphate ZAPP Heubach Zinc aluminum polyphosphateSAPP Heubach Strontium aluminum polyphosphate SRPP Heubach Modifiedstrontium aluminum polyphosphate ZCPP Heubach Modified zinc calciumpolyphosphate ZMP Heubach Zinc molybdate phosphate

The concentration of the leachable corrosion inhibitor in the bondingsolution may vary over a wide range depending on such factors as theidentity of the leachable corrosion inhibitor, the identity and amountof other components present, and the targeted properties of the coatingcomposition. For example, higher concentrations may be present incompositions that do not contain metal (e.g., aluminum) powder and/orwhere the leachable corrosion inhibitor is the only pigment present. Byway of example, the amount of leachable corrosion inhibitor may rangefrom about 2 to about 80 g, often from about 2 to about 50 g, per 100 mlof binder solution. In bonding solutions that are combined with aluminumpowder, the amount of leachable corrosion inhibitor often ranges fromabout 2 to about 15 g per 100 ml of binder solution.

It often may be desirable to avoid the use of chromate salts in theleachable corrosion inhibitors due to environmental considerations.However, chromate salts may be present in the leachable corrosioninhibitors in applications where such use can be tolerated. Thecompositions may contain other compatible known ingredients such assurfactants, wetting agents and other conventional additives.

It was found compositions exhibiting satisfactory stability andperformance characteristics can be prepared without the need for addingsources of zinc ions and/or aluminum ions as described in U.S. Pat. No.5,478,413. Relatively small quantities of ions such as zinc and aluminummay be present in the aqueous compositions as a result of equilibriumreactions involving metals contained in the leachable corrosioninhibitor. The bonding solution may also contain quantities of otherions, such as calcium and/or strontium ions. In some instances thebonding solution is free or substantially free of Fe ions and/or Mnions.

The composition may also contain borate ions, which may be supplied byany convenient form such as boron oxide, boric acid, and soluble boratesalts. Non-limiting examples of borate compounds are listed in theHandbook. Boron oxide hydrates into boric acid and then reacts formingmagnesium borate in solution.

The bonding solution may be prepared by first preparing a bindersolution by combining water, phosphoric acid, a source of magnesiumions, and optionally a source of borate ions. The binder solution isbuffered, e.g., to a pH of about 2 to about 3, often from about 2.4 toabout 2.7. The leachable corrosion inhibitor may then be added to thebuffered binder solution to prepare the bonding solution.

The coating slurry composition may be formed by mixing the abovedescribed bonding solution with the metal particles, e.g., aluminumparticles in the form of powder, flake, or a combination thereof. Thebonding solution is essentially inert with respect to any furtherreaction with the added aluminum. No visible reaction between the addedaluminum particles and the phosphoric acid is apparent in the coatingcomposition for at least one hour and in some instances for as long aseight hours or more.

The bonding solutions are particularly useful for forming coatingcompositions for ferrous metal alloy substrates when combined withparticulate aluminum. Particulate aluminums suitable for use in suchcoating compositions are well known, and have been discussed at lengthin the patent literature. For example, such particulate aluminums areset forth in Mosser U.S. Pat. Nos. 4,537,632, 4,544,408, 4,548,646,4,617,056, 4,659,613, and 4,863,516, which is particularly directed tothe use of non-leafing aluminum flake in combination with atomizedaluminum particles; and Mosser U.S. Pat. Nos. 4,889,558 and 5,116,672,all of which are incorporated herein by reference. A majority ofchromate/phosphate based compositions that utilize aluminum particlesuse atomized and/or flaked particles of various sizes for coatings withdifferent properties. These are of course also suitable for the presentbonding and coating compositions.

When aluminum is used in the compositions it may be gas atomizedspherical of an average size of 2.5-10 μm, air atomized of an averagesize of 4.5-10 μm, flake aluminum; flake/atomized mixtures; and aluminumalloys. Larger particles as well as smaller particles can be used.

The slurry coating compositions may be applied in a conventional way tothe ferrous metal alloy surface to be coated. Manners of application aredescribed in the patents referred to above and incorporated herein byreference. Generally, it is desirable to degrease the part to be coated,blast with aluminum oxide abrasive, and apply the coating by anysuitable means, such as by spraying, brushing, dipping, dip spinning,etc., drying until the color of the coating turns grayish, curing thecoating at a temperature of about 650° F. (343° C.) for 15 minutes orlonger, curing at higher or lower temperatures if desired. The slurry ispreferably applied in two coats or layers, each about 0.001 inch (25 μm)in thickness, then, if desired, dried at about 180° F. (82° C.) for 15to 30 minutes and then cured at 650° F. (343° C.) for 30 to 60 minutesafter each coat.

The coatings as cured at 650° F. (343° C.) are not electricallyconductive and therefore can not provide galvanic protection againstcorrosion of the underlying substrate material. However, the coating maybe made electrically conductive by burnishing with glass beads, abrasivemedia at low pressure or mechanically cold worked in other ways toproduce a conductive sacrificial coating or by heating as specified inMIL-C-81751B specification (incorporated herein by reference). In thismanner the coatings can, by mechanical or thermal processes, be madeelectrically conductive and thereby produce galvanic as well as barrierprotection of the underlying ferrous alloy substrate. Desirably, afterthe second layer is applied, dried, cured and processed to make itelectrically conductive, the surface of the coating may be sealed with abonding solution (seal coat) to further increase the oxidation andcorrosion protection provided by the coating, and to decrease the rateof consumption of aluminum in the coating during service. This bondingsolution can but need not be a bonding solution as described herein. Theseal coat may, in addition to having no additional fillers or pigments,contain pigments and fillers typical used in the industry. These includesuch materials as metal oxides such as alumina, silica, chromia, andtitania, as well as mixed metal oxides and oxide spinels such as copper,iron and manganese chromite, and magnesium ferrite. The purpose of thepigments may be to increase oxidation and corrosion protection as wellas provide improved application properties. The seal coats may be driedand cured at the same time and temperature as the above described slurrycoatings.

As has been described above, it is often desirable to provide bondingand coating compositions which are essentially free of chromate,molybdate and other like toxic or undesirable metals. In situationswhere more permissive environmental conditions would permit the use ofsuch metals as chromium, molybdenum, nickel and others, it is notexcluded that such metals be used in the bonding and/or the coatingcomposition. When chromium and/or molybdenum is present, the amount ofchromium and/or molybdenum should be less than that necessary topassivate or neutralize the phosphoric acid solution to the reactionwith metallic aluminum. The amount necessary to passivate the bondingsolution as taught in the prior art generally is at least 0.2% by weightof the final coating composition.

In some applications it is desirable to apply a protective topcoat tothe coated part, for example on turbine component surfaces which comeinto contact with the turbine gas path. Non-limiting examples oftopcoats include compositions containing phosphate and nitrate ions asdescribed in Myers et al. U.S. Pat. No. 5,968,240 and compositionscontaining phosphate and chromium III (Cr³⁺) ions as described in Myerset al. U.S. Pat. No. 6,224,657, the disclosures of which areincorporated herein by reference.

The following Examples are merely illustrative and should not beconstrued as limiting the invention.

EXAMPLE 1 Binder

A binder for a bonding solution was prepared by combining deionizedwater, phosphoric acid, boron oxide, and magnesium carbonate in theamounts listed in the table below.

Deionized Water 2000 ml 85% Phosphoric Acid 332 ml Boron oxide 41.6 gMagnesium Carbonate 225 g pH 2.4-2.6Bonding Solution

A bonding solution was prepared by mixing 100 ml of the binder with 4.2g of Heucophos ZPA. The resulting pH of this bonding solution was 3.1.

Coating Composition

A coating composition was prepared by mixing 100 ml of the bondingsolution with 70 g aluminum powder (5 μm).

EXAMPLES 2-9

Coating compositions were prepared in a manner described in Example 1from bonding solutions which had the following compositions.

Bonding Solution Binder Solution (per 100 ml binder solution) H₂O H₃PO₄B₂O₃ MgCO₃ ZPA KWhite ZAPP Ex. (ml) (ml) (g) (g) pH (g) 105 (g) (g) pH 22000 332 41.6 225 2.5 0 4.2 0 3.1 3 2000 332 41.6 225 2.5 0 0 4.2 3.2 42000 332 41.6 225 2.5 0 8.4 0 3.1 5 2000 332 41.6 225 2.5 0 0 8.4 3.1 62000 332 83.2 225 2.5 4.2 0 0 3.1 7 2000 332 0 225 2.5 4.2 0 0 3.2 82000 332 41.6 193 2.2 4.2 0 0 2.8 9 2000 332 41.6 225 2.5 8.4 0 0 3.3

COMPARATIVE EXAMPLES 1-4

Coating composition were prepared in a manner described in Example 1from bonding solutions which had the following compositions.

Comp. H₂O H₃PO₄ B₂O₃ MgCO₃ MnSO₄ Iron Pyrophosphate Zinc ZPA Ex. (ml)(ml) (g) (g) (g) (g) Oxide (g) pH 1 2000 920 124* 600 0 41 70 335 3.2 2800 183.6  8.4 73.6 53.2 0 8.4 42.2 2.8 3 2000 332  41.6 225 0 0 0 0 2.54 2000 920 124* 600 0 41 70 335 2.8 *added as boric acid

The coatings of Examples 1-9 and Comparative Examples 1-4 were testedfor the following criteria:

1. Pot life/stability of the mixed coatings at room temperature.

2. Oxidation corrosion resistance. 3×4 inch panels with one coat,0.8-1.0 mil thick (20-25 μm) were evaluated according to an industrystandard oxidation corrosion test. Panels were bead burnished, heattreated at 700° F. (371° C.) for 23 hours, then at 1075° F. (579° C.)for 4 hours, scribed, and placed in a 5% salt spray per ASTM B117 for400 hours.

3. Heat cycle salt spray. 4×4 inch panels with 2 coats, 2 cures, 1.5 mil(38 μm) thick, for 10 heat/salt spray cycles were evaluated where onecycle consisted of 7.5 hours at 850° F. (454° C.) and 15.5 hours in 5%salt spray. Coated panels were grit burnished and scribed prior totesting.

Test Procedure

Panels of 1010 mild steel were prepared by cutting 0.030 inch (0.75 mm)thick sheet stock and stamping each panel with a unique ID code. Thepanels were thermally degreased at 650° F. (343° C.) then grit blastedwith 100 mesh alumina grit at 60 psi. An oxidation step consisting of650° F. (343° C.) for one hour was performed prior to coatingapplication.

Results

Pot Life/Stability

Table I provides the results of the pot life/stability observations foreach coating. Comparative Example 3 had a very short pot life and showedsigns of reaction with the substrate. This demonstrates the adverseeffects in stability when a leachable corrosion inhibitor is notpresent.

TABLE I Time to bubbling Sprayability time based Example reaction (hr)On stirred viscosity (hr) 1 7 24-48 2   2.5 <24 3 4  30 4 3 <24 5   7.5 48 6  8† 120-144 7   2.5 <24 8 3 120-144 9 120  144 Comp. 1 7 24-48Comp. 2 2 <24 Comp. 3   0.25 1-3 Comp. 4   3.5  96-120 †initial reactionoccurred between 3 and 8 hoursOxidation Corrosion Resistance

Panels coated with Examples 1 through 9, all of which are formulatedaccording to the teachings of the invention, exhibited no red rust inthe scribe and minimal sacrificial (white) corrosion products in thefield (the coated area apart from the scribe). The coating ofComparative Example 3, which was formulated with no corrosion inhibitor,exhibited red rust in the scribe and significantly more sacrificialcorrosion in the field than coatings of Ex. 1 to 9. This behaviordemonstrates that a leachable corrosion inhibitor improves corrosionresistance of the cured coating as well as the pot life of the mixedcoating as discussed above. Significant red rust was observed in thescribe of the panels coated with Comparative Example 2, demonstratingthat the corrosion benefits of a leachable corrosion inhibitor can beeclipsed by other ions in the coating, e.g. manganese and sulfate.

Heat Cycle Salt Spray

The panels of Comparative Example 2 showed red rust in the field and inthe scribe as early as 4 cycles into the test. After 5 cycles thiscoating began to delaminate in big pieces, particularly on the backs ofthe panel. All other specimens had some red rust in the scribe but nofield rust, and some signs of sacrificial behavior (discoloration andwhite corrosion products). The panels of Example 4 had more corrosion inthe scribe and at the upper edge than other panels. Coatings of Examples1, 2, 3, 5, 6, 7, 8, and Comparative Examples 1, 3, and 4 were judged tohave similar performance in this test.

While particular aspects have been described, it should be understoodthat the invention is not limited thereto since modifications may bemade by persons skilled in the art. The present application contemplatesany and all modifications that fall within the spirit and scope of theunderlying invention disclosed and claimed herein.

1. An aqueous bonding solution consisting essentially of phosphoricacid, a source of magnesium ions, and a leachable corrosion inhibitor,wherein the solution has a pH from about 2 to about 4.5 and issubstantially free of chromate ions.
 2. The bonding solution of claim 1,wherein the source of magnesium ions is selected from the groupconsisting of magnesium oxide, magnesium carbonate, and a combinationthereof.
 3. The bonding solution of claim 1, wherein the leachablecorrosion inhibitor is selected from the group consisting of zincaluminum orthophosphate, zinc aluminum tripolyphosphate, aluminumtripolyphosphate, zinc aluminum polyphosphate, strontium aluminumpolyphosphate, modified strontium aluminum polyphosphate, modified zinccalcium polyphosphate, zinc molybdate phosphate, and combinationsthereof.
 4. The bonding solution of claim 1, wherein the solution issubstantially free of molybdate and vanadium ions.
 5. The bondingsolution of claim 1, wherein the solution has a pH from about 2.5 toabout 3.5.
 6. An aqueous phosphoric acid coating composition comprisingthe bonding solution of claim 1 and metallic particles.
 7. The coatingcomposition of claim 6 which is stable with respect to reaction with themetallic particles for at least one hour.
 8. The coating composition ofclaim 7 which is stable for at least four hours.
 9. The coatingcomposition of claim 6 wherein the metallic particles comprise aluminumpowder, aluminum flake, or a combination thereof.
 10. A method ofcoating a part having a ferrous alloy surface comprising applying thecoating composition of claim 6 to a surface of the part and subjectingthe part to heat to cure the coating.
 11. A coated part having a metalsurface coated with a cured coating composition of claim
 6. 12. Thecoated part of claim 11 further comprising a protective topcoat.
 13. Anaqueous bonding solution consisting essentially of phosphoric acid; asource of magnesium ions selected from the group consisting of magnesiumoxide, magnesium carbonate, and a combination thereof; and a leachablecorrosion inhibitor selected from the group consisting of zinc aluminumorthophosphate, zinc aluminum tripolyphosphate, aluminumtripolyphosphate, zinc aluminum polyphosphate, strontium aluminumpolyphosphate, modified strontium aluminum polyphosphate, modified zinccalcium polyphosphate, zinc molybdate phosphate, and combinationsthereof; wherein the solution has a pH from about 2.5 to about 3.5. 14.The bonding solution of claim 13, wherein the pH is from about 2.7 toabout 3.3.
 15. The bonding solution of claim 13, wherein the leachablecorrosion inhibitor is selected from the group consisting of zincaluminum orthophosphate, zinc aluminum tripolyphosphate, aluminumtripolyphosphate, and combinations thereof.
 16. An aqueous phosphoricacid coating composition comprising the bonding solution of claim 13 andmetallic particles.
 17. A method of coating a part having a ferrousalloy surface comprising applying the coating composition of claim 13 toa surface of the part and subjecting the part to heat to cure thecoating.
 18. A coated part having a metal surface coated with a curedcoating composition of claim
 13. 19. The coated part of claim 18 furthercomprising a protective topcoat.
 20. An aqueous phosphoric acid coatingcomposition comprising the bonding solution of claim 1 and at least onenon-metallic pigment.
 21. The coating composition of claim 20 whereinthe at least one non-metallic pigment is selected from the groupconsisting of alumina, zirconia, ceria, and mixed metal oxides.
 22. Anaqueous bonding solution consisting essentially of phosphoric acid, asource of magnesium ions, a source of borate ions, and a leachablecorrosion inhibitor, wherein the solution has a pH from about 2 to about4.5.
 23. The bonding solution of claim 22, wherein the source of borateions is selected from the group consisting of boron oxide and boricacid.